System for on-line and off-line decryption

ABSTRACT

A secure communication system wherein message decryption may be performed while off-line, or optionally while on-line. A sender encrypts a message based on the message key and sends it to the recipient. An envelope containing a message key is created by encrypting the message key based on a verifier, where the verifier is based on a secret of the recipient. The recipient is provided the envelope, along with the message or separately, from the sender or from another party, contemporaneous with receipt of the message or otherwise. The recipient can then open the envelope while off-line, based on their secret, and retrieve the message key from the envelope to decrypt the message. In the event the recipient cannot open the envelope, optional on-line access permits obtaining assistance that may include obtaining an alternate envelope that the recipient can open.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/449,068, filed Feb. 20, 2003.

BACKGROUND OF INVENTION

1. Technical Field

The present invention relates generally to secure electroniccommunication and more particularly to encryption and decryption ofe-mail and other messages, files or other information.

2. Background Art

A key server may be used for managing and distributing symmetricencryption keys, that is, keys for an encryption system in which theencryption key and the decryption key for a particular message are thesame. For example, in a secure e-mail system, a sender of an e-mail mayrequest that the key server create and store a message key, that is, anencryption/decryption key for a message that is unique to thatparticular message or unique for a particular series of messages. Thesender then encrypts the e-mail with the message key and sends it to therecipients. A given recipient then requests the message key from the keyserver, which determines the authenticity of the recipient. If therecipient is authentic and is also authorized to receive the message key(as specified by the original sender), the key server delivers themessage key to the recipient, which uses the message key to decrypt thee-mail.

Distributing symmetric keys via a key server has many positiveattributes. For example, a sender (or any authorized party) candetermine when a recipient has requested and received the message key.This “key advisement” can form the basis of an audit system. Also, asender (or any authorized party) can control access to the message key,including specifying not-before and not-after delivery times for a key.In this way, the message key can be made available only during a certaintime window, or access can be terminated if conditions warrant denyingany further access to the message.

Most present key server schemes make off-line decryption impossiblebecause they require that the recipients be on line to communicate withthe key server. There are some exceptions to this, however, and theseoff-line decryption systems generally use key enveloping via one of thefollowing schemes. First, a sender can encrypt a message with a messagekey that is chosen at random. The message key is then encrypted (i.e.,enveloped) with another key that is derived from a password known to thesender and all of the recipients. Second, as above, except that themessage key is encrypted with a public key of the recipient. In eithercase, there is typically one envelope per recipient, particularly in thesecond scheme where each recipient's public key is different.

The first scheme above is weak. Enveloping a message key with anotherkey that is derived from a password is susceptible to off-linedictionary attacks on the password. Given that most passwords need to bememorized by human users, and given that passwords must consist ofprintable characters, the effective length of a key derived from apassword is anywhere from 1.5 to 5 bits per character. Thus, theeffective length of a key derived from a twelve character password(which has 50% more characters than a typical password of eightcharacters) is anywhere from 18 to 60 bits. By today's standards, such akey is very weak and is subject to brute force attacks. In summary, akey derived from a password is subject to both off-line dictionaryattacks as well as brute force attacks.

The second scheme above is very strong. However, enveloping a messagekey with the recipient's public key imposes burdensome requirements. Forexample, all intended recipients must already have a public key, andthose must be available to the sender at the time of enveloping. Incases where the sender and recipients are new to each other, simplyascertaining public keys can be an obstacle. Setting up, by obtainingpublic and private keys and such, can also be daunting when a recipientis new to the scheme. Not surprisingly, many potential recipients optout if any other options exist, even less secure ones, and many resistadoption until they expect to receive substantial numbers of messagessecured in this manner. Furthermore, the private key of each recipientmust be available at the place where that recipient desires to read themessage. For instance, if a recipient stores his private key at acomputer at work, he would not be able to decrypt the message at a homecomputer that does not also have a copy of the recipient's private key.

In summary, a password-based scheme is easy to use but offers weaksecurity. A public key scheme offers strong security but is verydifficult to deploy and use. Because of the reasons mentioned above, thecurrent state-of-the-art off-line decryption systems do notsimultaneously satisfy both security and ease-of-use requirements.

SUMMARY OF INVENTION

Accordingly, it is an object of the present invention to provide asecure communication system that can simultaneously satisfy requirementsof high security and high ease of use.

Briefly, one preferred embodiment of the present invention is a systemfor secure communication of a message from a sender to a recipient. Anenvelope is created containing a message key, by encrypting the messagekey based on a verifier that is based on a secret of the recipient. Themessage key is provided to the sender, where the message is encryptedbased on the message key. The message is sent from the sender to therecipient. The envelope is also provided to the recipient, typically butnot necessarily along with the message. The recipient then open theenvelope. This is done based on the secret of the recipient, and therecipient is then able to retrieve the message key from the envelope anddecrypt the message based on the message key.

Briefly, another preferred embodiment of the present invention is asystem for a sender to encrypt a message intended for a recipient. Amessage key is provided. Then an envelope is created containing themessage key, by encrypting the message key based on a verifier that isbased on a secret of the recipient. The message is encrypted, based onthe message key. This then permits the message to be sent securely fromthe sender to the recipient and, when the recipient is provided with theenvelope, typically but not necessarily along with the message, thesecret can be used to open the envelope to retrieve the message key anddecrypt the message.

Briefly, one preferred embodiment of the present invention is a systemfor secure communication of a message from a sender to a recipient. Anenvelope is created containing a message key, by encrypting the messagekey based on a verifier that is based on a secret of the recipient. Themessage key is provided to the sender, where the message is encryptedbased on the message key. The message is sent from the sender to therecipient. The envelope is also provided to the recipient, typically butnot necessarily along with the message. The recipient then opens theenvelope. This is done based on the secret of the recipient, and therecipient is then able to retrieve the message key from the envelope anddecrypt the message based on the message key.

An advantage of the present invention is that it provides both highsecurity and high ease of use. With respect to improved security, thepresent invention uses encryption of message keys (enveloping) based ona verifier, rather than relying upon an envelope key derived directlyfrom a password and the inherent weakness such introduces. With respectto improved ease of use, the present invention uses such enveloping anddecryption (de-enveloping or envelope opening) to access the message keybased on a corresponding secret, rather than a more complex scheme likepublic key infrastructure (PKI).

And another advantage of the invention is that embodiments of theinvention optionally employ a mixture of on-line and off-line decryptioncapabilities, further combining high security high flexible utility.

These and other objects and advantages of the present invention willbecome clear to those skilled in the art in view of the description ofthe best presently known mode of carrying out the invention and theindustrial applicability of the preferred embodiment as described hereinand as illustrated in the several figures of the drawings.

BRIEF DESCRIPTION OF DRAWINGS

The purposes and advantages of the present invention will be apparentfrom the following detailed description in conjunction with the appendedfigures of drawings in which:

FIG. 1 (background art) is a functional block diagram of an on-linesecure communication system.

FIGS. 2A-C (background art)(extending across three sheets) is a networkdata flow diagram of an example message encryption, sending, anddecryption that occurs within the secure communication system of FIG. 1.

FIG. 3 is a functional block diagram of an online/off-line securecommunication system according to the present invention.

FIGS. 4A-B (extending across two sheets) is a network data flow diagramof an example message encryption, sending and decryption process thatoccurs within the improved secure communication system of FIG. 3.

In the various figures of the drawings, like references are used todenote like or similar elements or steps.

DETAILED DESCRIPTION

A preferred embodiment of the present invention is a system for on-andoff-line decryption in the greater context of a secure communicationsystem. As illustrated in the various drawings herein, and particularlyin the view of FIG. 3, a preferred embodiment of the invention isdepicted by the general reference characters 130.

TERMINOLOGY

Unless stated otherwise, the following terminology is used herein.

Message key, encryption key, decryption key, or simply the key mean thesymmetric key that is used to encrypt or decrypt a message.

Message means the unit of data that is encrypted and decrypted.Throughout this document we use e-mail as an example of a message.However, other kinds of messages are also envisioned. These includeinstant messages, chat messages, messages communicated between twoapplications using a protocol other than e-mail (SMTP) and manners oftransferring files other than as e-mail attachments (e.g., FTP), etc.

Sender means the encryptor of the message.

Recipient, sometimes called receiver, means the decryptor of themessage. The list of recipients can include the sender, or even besolely the sender. This is the case when a person encrypts a message forsecure communication or storage so that only he or she can decrypt itlater.

Envelop key means the symmetric key that encrypts/decrypts the messagekey, wherein an envelop encryption key is the public key that encryptsthe message key and an envelop decryption key is the private or secretkey that decrypts the message key.

Key exchange algorithm means the algorithm a sender and the recipientsuse to derive the envelop key.

Key encryption algorithm means the algorithm the sender and recipientsuse to encrypt or decrypt the envelop key.

Session key means an encryption/decryption key that is used to secureon-line communication between various components of the system. Sessionkeys are preferably temporary and not stored on any server.

A Background Art on Line Encryption/Decryption System

FIG. 1 (background art) is a functional block diagram of a securecommunication system 100 that the present invention improves upon. Thesecure communication system 100 here consists of three major components:clients 102, an authentication server 104, and a key server 106. Theclients 102 are conceptually viewed as one component because senders 108and recipients 110 collectively are both “clients” of the authenticationserver 104 and key server 106. All interactions between the clients 102(that is, either a sender 108 or a recipient 110) and the authenticationserver 104 or the key server 106 may be encrypted using short-livedsession keys.

FIGS. 2A-C (background art)(with parts A through C extending acrossthree sheets) is a network data flow diagram of an example messageencryption, sending, and decryption that occurs within the securecommunication system 100. Each of FIGS. 1 and 2A-C show the processactivities associated with the major components of the securecommunication system 100 for encryption and decryption of an examplemessage 112. These process activities are as follows.

A1: The sender 108 authenticates by sending an authentication request114 to an authentication server 104.

A2: The authentication server 104 authenticates the sender 108 viawhatever method is appropriate. Various methods can be supported, andmultiple ones can be supported concurrently. Which particular method isused, however, is not particularly germane here. Upon successfulauthentication, the authentication server 104: creates a digitallysigned sender assertion 116, vouching for the identity of the sender108.

A3: Subject to successful authentication, the authentication server 104sends the sender assertion 116 to the sender 108.

A4: The sender 108 sends a sender key request 118 to the key server 106.The sender key request 118 includes the sender assertion 116 and arecipient list 120 of authorized recipients 110 of the message 112, andformally requests a message key 122.

A5: The key server 106 validates the sender assertion 116, creates themessage key 122, and stores the message key 122 along with the recipientlist 120 in an internal database.

A6: The key server 106 sends the message key 122 to the sender 108.

A7: The sender 108 encrypts the message 112 using the message key 122.

A8: The sender 108 sends the encrypted message 112 to the recipients110. There may be many intermediary relays (not shown in the figures)between the sender 108 and each recipient 110. These intermediariessimply relay the message 112 but are not privy to the message key 122,unless a particular intermediary also happens to be a recipient 110 ofthe message 112.

A9: The recipient 110 sends an authentication request 124 to anauthentication server 104. The authentication server 104 with which arecipient 110 authenticates may be, but need not be, the same as theauthentication server 104 with which the sender 108 authenticates. Theauthentication request 124 here can be substantially the same as anauthentication request 114 that a sender 108 authenticates with, butthat is not a requirement and different criteria may apply.

A10: The authentication server 104 authenticates the recipient 110 viawhatever method is appropriate. Again, various methods can be supported.Upon successful authentication, the authentication server 104 creates adigitally signed recipient assertion 126, vouching for the identity ofthe recipient 110. The recipient assertion 126 here can also besubstantially the same as a sender assertion 116 vouching for theidentity of the sender 108, but that is also not a requirement.

A11: Subject to successful authentication, the authentication server 104sends the recipient assertion 126 to the recipient 110.

A12: The recipient 110 sends a recipient key request 128 to the keyserver 106. The recipient key request 128 includes a resource ID (whichuniquely identifies the decryption key) and the recipient assertion 126,and formally requests the message key 122.

A13: The key server 106 validates the recipient assertion 126, checksits internal database to confirm that the recipient 110 is in therecipient list 120, and retrieves the message key 122.

A14: The key server 106 sends the message key 122 to the recipient 110.

A15: The recipient 110 uses the message key 122 to decrypt the message112.

There must exist an a-priori trust relationship between theauthentication server 104 (or authentication servers 104, if more thanone is employed) and the key server 106. That is, the key server 106must trust the authentication server 104 to vouch for the identity of aset of clients 102. Said another way, the key server 106 must verifythat the assertions the clients 102 provide to the key server 106 havebeen created by the authentication server 104 and have not beenmodified. The key server 106 can implement this trust relationship byacquiring a public verification key of the authentication server 104(e.g., a X.509 certificate of the authentication server 104, bearing itspublic key). The authentication server 104 can then use itscorresponding private key to sign the assertions 116, 126.

The secure communication system 100 shown in FIG. 1 requires that thesender 108 and all of the recipients 110 be on-line to receive themessage key 122, though it is not required that the sender 108 and anyrecipient 110 be on-line at the same time.

Adding Off-Line Decryption

We now describe how the secure communication system 100 just describedcan be extended to also provide an off-line decryption capabilitywhereby, subsequent to receipt of an encrypted message, a recipient neednot communicate with any other component in order to decrypt themessage. Suitable embodiments of the invention can also provide on-linedecryption capability when off-line decryption is not possible (e.g.,when a recipient has forgotten his or her password). And suitableembodiments can enable a sending organization to implement a policy thatsatisfies on-line and off-line decryption requirements on aper-recipient basis.

Off-line decryption relies on an encryptor having access to eachrecipient's verifier. A verifier is analogous to a public key. However,instead of a having a random public/private key pair, a verifier iscreated based on a known secret (typically, a password). Verifiers areknown in the art; see for example, the Secure Remote Password (SRP)proposed by THOMAS WU in IETF RFC 2945, “The SRP Authentication And KeyExchange System”. A party who knows a verifier can challenge a party whoclaims to know the corresponding secret. However, the secret need not bedivulged to the challenging party. Nor is it feasible for any party thatknows the verifier to guess the corresponding secret.

FIG. 3 is a functional block diagram of a secure communication system130 according to the present invention. The secure communication system130 consists of three major components: clients 102, an authenticationserver 104, and a key server 106. The clients 102 are again conceptuallyviewed as one component because the senders 108 and recipients 110collectively are both “clients” of the authentication server 104 and thekey server 106. The authentication server 104 may be the same as in thesecure communication system 100 of FIGS. 1 and 2A-C (background art).The key server 106 now has additional capabilities, however. And, asdiscussed presently, a key server 106 may also be used in embodiments ofthe invention that operate in the manner of the secure communicationsystem 100 and alternately in the manner of the secure communicationsystem 130. Also, as was the case for the secure communication system100, all interactions between a client 102 (that is, either a sender 108or a recipient 110 and either the authentication server 104 or the keyserver 106 may be encrypted using short-lived session keys.

FIGS. 4A-B (in parts A and B extending across two sheets) is a networkdata flow diagram of an example message encryption, sending anddecryption process that occurs within the secure communication system130. Each of FIGS. 3 and 4A-B show the process activities associatedwith the major components of the secure communication system 130 forencryption and decryption of an example message 112. These processactivities are as follows.

B1: The sender 108 authenticates by sending an authentication request114 to an authentication server 104

B2: The authentication server 104 authenticates the sender 108 viawhatever method is appropriate (various and multiple methods can besupported for this). Upon successful authentication, the authenticationserver 104 creates a digitally signed sender assertion 116, vouching forthe identity of the sender 108.

B3: Subject to successful authentication, the authentication server 104sends the sender assertion 116 to the sender 108.

B4: The sender 108 sends a sender key request 118 to the key server 106.The sender key request 118 includes the sender assertion 116 and arecipient list 120 of authorized recipients 110 of the message 112, andformally requests a message key 122.

Activities B1 through B4 may be essentially the same as activities A1through A4, described with respect to FIGS. 1 and 2A-C.

B5: The key server 106 validates the sender assertion 116, creates themessage key 122, and places the message key 122 in an envelope 132. Eachmessage 112 may have one or more message keys 122. For instance,multiple message keys 122 might be used when a message 112 has multipleparts like a body and one or more attachments. Each message key 122 mayalso be put in multiple envelopes 132, usually one per recipient 110. Asingle envelope 132 might also be used for multiple recipients 110, butthat is generally not desirable because each recipient 110 would thenhave to know the corresponding secret(s) that opens the envelope 132.Additionally, in a typically used option that is discussed furtherpresently, the key server 106 can also store the message key 122 alongwith the recipient list 120 in an internal database.

B6: The key server 106 sends the message key 122 and all of theenvelopes 132 (each containing an encrypted copy of the message key 122)to the sender 108.

B7: The sender 108 encrypts the message 112 with the message key 122.

B8: The sender 108 sends the encrypted message 112 along with theenvelopes 132 to the recipients 110. All of the recipients 110 can besent all the envelopes 132 (which are generally small), or traffic canbe reduced by providing each recipient 110 with only the envelope 132 itwill need. There may be many intermediary relays between the sender 108and the recipient 110 (not shown in the figures). The intermediariessimply relay the message 112 but are not privy to the message key 122 orthe contents of any envelope 132, unless an intermediary also happens tobe an authorized recipient 110 of the message 112.

Activities B5 through B8 are modified from activities A5 through A8,described with respect to FIGS. 1 and 2A-C.

B9: The recipient 110 uses the secret 136, corresponding with theverifier 134, to open (decrypt) the appropriate envelope 132 to obtainthe message key 122.

B10: The recipient 110 uses the message key 122 to decrypt the message112.

Activity B9 replaces activities A9 through A14 and activity B10 may beessentially the same as activity A15, as described with respect to FIGS.1 and 2A-C.

Creating the Verifier

The secure communication system 130 just described uses the verifier 134to create the encrypted envelopes 132, which contain the message key122. There are multiple methods by which the key server 106 can know theverifier 134 for each recipient 110, five of which are described below.Also, each envelope 132 could use a different method; that is,enveloping for all recipients 110 need not use the same method.

First, the key server 106 may ask the authentication server 104 for averifier 134 for each recipient 110. In this case, one or more of thefollowing may apply. The authentication server 104 may already have theverifier 134; the authentication server 104 may have the secret 136 ofthe recipient 110, and thus be able to create the verifier 134 on thefly; or the authentication server 104 may have data that is equivalentto the secret 136 (e.g., a hash of the secret 136), and can create theverifier 134 on the fly from this.

Second, the key server 106 may create the verifier 134 on the fly byasking the authentication server 104 for the secret 136 of the recipient110, or for data that is equivalent to it (e.g., a hash of it). Third,the sender 108 can provide the verifier 134 of a recipient 110 to thekey server 106, based on a-priori knowledge of the verifier 134. Fourth,the sender 108 can create the verifier 134 of a recipient 110 on the flyand provides it to the key server 106. And fifth, the key server 106 cancreate the verifier 134 on the fly, based on the secret 136 which thesender 108 provides.

The sophisticated variations of the secure communication system 130described above use the key server 106, but even this is not arequirement. The sender 108 can have or create the verifier 134, andthen use it itself to create the envelope 132. The sender 108 can dothis using a message key 122 obtained from a key server 106, with orwithout involvement of an authentication server 104, or the sender 108can have or create the message key 122.

The Enveloping Algorithm

There are various possible methods for creating the envelope 132containing the message key 122, two of which are now discussed. First,the verifier 134 can be used to create an envelope key. One suitabletechnique for this is to derive the envelop key via the publicly-knownDiffie-Hellman key agreement. For example, the creator of the envelopekey may use the verifier 134 to arrive at, say, some 2,000 bits of data,wherein the recipient 110 will be able to arrive at those same 2,000bits of data by using the secret 136. Then, a conventional encryptionalgorithm (e.g., AES) can be used to encrypt the message key 122 withthe envelop key, thereby creating the envelope 132. This requires thecreator of the envelope 132 to include how the envelop key was derivedand what algorithm was used to encrypt the message key 122. Continuingwith our example, since only, say, 128 bits are needed by the encryptionalgorithm, some accord or advisement is needed whereby the recipient 110will know which 128 bits out of the available 2,000 bits the envelopekey creator used and, furthermore, which encryption algorithm was used.

Second, the verifier 134 can be more directly used to create theenvelope 132 itself. That is, an encryption key for the envelope 132 canbe based on the verifier 134 and a corresponding decryption key for theenvelope 132 can be based on the secret 136 corresponding to theverifier 134 This method has the advantage that the creator of theenvelope 132 need not specify how the encryption key for the envelope132 was derived. One example technique suitable for this is to encryptthe message key 122 via the publicly-known El-Gamal encryptionalgorithm.

Some Alternative Embodiments

We now consider various alternative embodiments of the invention, someof which include a combination of aspects of the secure communicationsystems 100, 130 described above, and others of which build uponrespective aspects of the secure communication systems 100, 130.

On-line key retrieval, e.g., in the manner of the secure communicationsystem 100, and off-line decryption, e.g., in the manner of the securecommunication system 130, are not mutually exclusive. On-line keyretrieval can be used as a fallback mechanism. As noted when discussingactivity B5, above, the key server 106 can store the message key 122 inits database. In the case that a recipient 110 cannot open the envelope132, say, because the recipient 110 has forgotten the secret 136corresponding to the verifier 134 that was used to create the envelope132, the recipient 110 can be given the option to communicate with thekey server 106 and request the message key 122.

The sender 108 can communicate a key retrieval policy to the key server106 to indicate exactly how each recipient 110 can retrieve the messagekey 122. For example, a sender 108 can specify a set of recipients 110that must get the message key 122 by retrieving it from the key server106 (i.e., be on-line and request the message key 122 from the keyserver 106), and the sender 108 can also specify a set of recipients 110that can be off-line. The key server 106 creates and stores the messagekey 122. Additionally, the key server 106 can create the envelopes 132for only the set of recipients 110 who are authorized to decrypt themessage 112 off-line. Similarly, any authorized party (e.g., the keyserver 106 itself, an administration client of the key server 106, etc.)can set the key retrieval policy.

In cases where the key server 106 does not have access to the verifiers134 of recipients 110, the sender 108 can create the envelopes 132 andinclude them in the message 112. Note that in such a case, the keyserver 106 operates in the manner of the secure communication system100, i.e., in an on-line mode. It is then the sender 108 that, uponreceiving the message key 122, creates the envelopes 132 and includesthem when sending the message 112.

There may also be a desire to eliminate the key server 106 all together,or to simply not use it. This is particularly advantageous in the caseof peer-to-peer communication, consisting of small sets of senders 108and recipients 110. In such embodiments of the invention, the sender 108creates the message key 122 and the envelopes 132. There is no on-linekey retrieval capability if no key server 106 exists, or when a keyserver 106 does exist but has not been employed and does not have themessage key 122.

In a typical embodiment, the invention may employ the authenticationserver 104 as the custodian of the verifiers 134, since it can easilycreate and store the verifiers 134 for its existing users (i.e.,potential recipients 110. To make this easy and transparent, it can bedone whenever the authentication server 104 solicits a user's privatecredentials for any reason, including ones that have nothing to do withcreating assertions 116, 126 for accessing the key server 106. Typicallya password is the credential or “secret” that is used. Furthermore, oncethe authentication server 104 has created and stored a verifier 134, itcan update it whenever a user changes their private credentials. Thishas two benefits. First, it makes creation of the verifier 134transparent (though, users could be given notice of such an action iftheir agreement is required). Second, the verifier 134 can be updatedtransparently when a user changes their secret 136.

A verifier 134 is typically constructed from a secret 136 that is apassword. However, this need not be the case. A verifier 134 can also beconstructed from any number of attributes of the recipient 110, eitherpublic or private. For example, a verifier 134 could be constructedbased on a Social Security number, mother's maiden name, state ofresidence, etc. The strength of the verifier 134 is proportional to thenumber and secretive strengths of the attributes that go into itsconstruction.

As mentioned previously, in some embodiments of the invention, theauthentication server 104 may be the custodian of the verifiers 134However, because verifiers 134 are generally public data, they need notbe stored in a trusted repository. Thus, yet other embodiments of theinvention can use a verifier repository that is separate from theauthentication server 104

An important limitation of an off-line decryption system is thatoff-line decryption is not possible if a recipient 110 forgets his orher secret 136. Moreover, if the recipient 110 changes the secret 136,all messages 112 enveloped using the old secret 136 cannot be openedusing the new secret 136. As a result, the recipient 110 must remembermultiple secrets 136 (e.g., multiple passwords).

Some embodiments of the invention overcome these limitations using thefollowing method. When a recipient 110 has changed the secret 136 hemust go on-line to retrieve the message key 122. Once on-line, the keyserver 106 can create a new envelope 132 (based on the current verifier134 for the current secret 136 of the recipient 110 and send thatenvelope 132 to the recipient 110. This allows for a reasonably seamlessroll-over of secrets 136 of the recipient 110. However, a limitation ofthis is that the recipient 110 must be on-line once for every message112 having a verifier 134 that no longer matches the current secret 136.The key server 106 could send multiple envelopes 132 using the newverifier 134. For example, if a user has 100 messages 112 where themessage keys 122 were enveloped using an old verifier 134, once on-line,the recipient 110 can get the new envelopes 132 from the key server 106for all 100 of the previous message keys 122 (or, even one envelope 132containing the 100 message keys 122).

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of the invention should not belimited by any of the above described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

INDUSTRIAL APPLICABILITY

The present secure communication system 130 is well suited forapplication in electronic communications of e-mail, other message types,files, and other information, concurrently providing both high securityand high ease of use for both on-line and off-line decryption.

Unlike the majority of prior art schemes, the present invention permitsoff-line decryption by message recipients. Alternately, the presentinvention can also permit on-line decryption, establishing this as arequirement for some of multiple recipients or providing it as a fallback, for instance, when a recipient forgets their password.

Further, unlike prior art off-line decryption schemes that useenveloping where a message key is encrypted based on an envelope keyderived directly from a password, and the notorious attendantsusceptibility of such to various types of attacks on the password, thepresent invention uses encryption based on a verifier that correspondswith a secret of the message recipient. Such verifiers may be madeconsiderably more substantial than passwords, yet the correspondingsecrets can be passwords, and thus can be easily remembered and used bythe recipients.

Furthermore, unlike other prior art off-line decryption schemes that usecomplex arrangements like public key infrastructure (PKI) wherein largepublic keys must be ascertained, procured, stored, and availablewhenever and wherever one wishes to send or read a secured message, thepresent invention again uses the verifier/secret based approach whereboth the verifier and the secret are easily used by the respectiveparties employing them. While a verifier is analogous to a public key,it is far less odious to use. Similarly, a secret is (remotely)analogous to a private key, and far less odious to use. Since a secretcan be a password, or based on some other public or private attribute ofthe recipient, it is quite easy for recipients to remember and work withsecrets.

Nonetheless, while providing the noted and other advantages, the presentinvention may now be implemented by those of reasonable skill in theart, creating embodiments using existing technologies if desired, andthen used by individuals and organizations with ordinary skills andaptitudes.

For the above, and other, reasons, it is expected that the securecommunication system 130 of the present invention will have widespreadindustrial applicability. Therefore, it is expected that the commercialutility of the present invention will be extensive and long lasting.

1. A method for secure communication of a message from a sender to arecipient, the method comprising the steps of: in a first computerizedsystem, creating an envelope containing a message key by encrypting saidmessage key based on a verifier that is based on a secret of therecipient; providing said message key to the sender; at the sender, in asecond computerized system, encrypting the message based on said messagekey; sending the message from the sender to the recipient; providingsaid envelope to the recipient; and at the recipient, in a thirdcomputerized system: opening the envelope based on said secret of therecipient; retrieving said message key from the envelope; and decryptingthe message based on said message key.
 2. A method for a sender toencrypt a message intended for a recipient, the method comprising thesteps of: (a) providing a message key; (b) in a first computerizedsystem, creating an envelope containing said message key by encryptingsaid message key based on a verifier that is based on a secret of therecipient; and (c) in a second computerized system, encrypting themessage based on said message key, thereby permitting the message to besent securely with said envelope from the sender to the recipient, andthe recipient to be provided said envelope so that said secret can beused to open said envelope to retrieve said message key and decrypt themessage.
 3. The method of claim 2, wherein said step (a) includesgenerating said message key at the sender itself.
 4. The method of claim2, wherein said step (a) includes: obtaining said message key at a keyserver; and the sender receiving said message key from said key server.5. The method of claim 4, wherein said key server stores a copy of saidmessage key.
 6. The method of claim 5, wherein said step (a) includesinstructing said key server whether and under what conditions saidmessage key may be released to parties other than the sender itself. 7.The method of claim 5, wherein said step (a) includes: the senderproviding a recipient list to said key server; and said key serverstoring a copy of said recipient list.
 8. The method of claim 4, whereinsaid step (a) includes authenticating the sender as a condition of saidkey server providing said message key.
 9. The method of claim 8, whereinsaid step (a) includes the sender submitting a sender assertion to thekey server, wherein said sender assertion originates from anauthentication server.
 10. The method of claim 2, wherein said step (b)includes: deriving said envelope key based on a key agreement protocol;and encrypting said message key using a symmetric encryption algorithm.11. The method of claim 10, wherein: said key agreement protocol is theDiffie-Hellman key agreement; and said encryption algorithm is the AESencryption algorithm.
 12. The method of claim 2, wherein said step (b)includes encrypting said envelope key directly with said verifier,thereby permitting decrypting said envelope directly with said secret.13. The method of claim 12, wherein said step (b) includes encryptingsaid message key based on a public key encryption algorithm.
 14. Themethod of claim 13, wherein said public key encryption algorithm is theEl-Gamal encryption algorithm.
 15. The method of claim 2, wherein saidstep (b) includes generating said envelope at the sender itself.
 16. Themethod of claim 2, wherein said step (b) includes: generating saidenvelope at a key server; and the sender receiving said envelope fromsaid key server.
 17. The method of claim 16, wherein: the recipient isone of a plurality of recipients of the message; and said step (b)includes the sender instructing said key server which of said pluralityof recipients said key server is to create said envelopes for, therebyimplementing a policy that at least some of said plurality of recipientsmust go on-line to get said message key while others may read themessage off-line.
 18. The method of claim 16, wherein said step (b)includes said sender providing either said verifier or said secret tosaid key server, thereby permitting said key server to create saidverifier.
 19. The method of claim 18, wherein said step (b) includessaid sender creating and providing said verifier to said key server. 20.The method of claim 16, wherein said step (b) includes said key serverasking an authentication server for either said verifier or said secret,thereby permitting said key server to create said verifier.
 21. Themethod of claim 20, wherein said authentication server employs a memberof the set consisting of already having said verifier, having saidsecret and creating said verifier, having data equivalent to said secretand creating said verifier, and having a hash of said secret andcreating said verifier.
 22. The method of claim 2, wherein said secretis a password.
 23. The method of claim 2, wherein said secret is basedon at least one public or private attribute of the recipient other thana password.
 24. The method of claim 2, wherein the verifier is arecipient verifier and the sender includes a sender verifier with themessage, thereby permitting the recipient to easily reply to the messagein a secure manner.
 25. The method of claim 24, wherein said senderverifier is included in said envelope.
 26. A system for a sender toencrypt a message intended for a recipient, comprising: a firstcomputerized system able to create an envelope containing a message keyby encrypting said message key based on a verifier that is based on asecret of the recipient; said first computerized system further able toprovide at least said envelope to a second computerized system, whereinsecond computerized system is employed by the sender; and said secondcomputerized system able to encrypt the message based on said messagekey, thereby permitting the message to be sent securely from the senderto the recipient and the recipient to be provided said envelope so thatsaid secret can be used to open said envelope to retrieve said messagekey and decrypt the message.
 27. The system of claim 26, wherein saidfirst computerized system and said second computerized system are thesame.
 28. The system of claim 26, wherein: said first computerizedsystem is a key server; and said second computerized system receivessaid message key from said key server.
 29. The system of claim 28,wherein said key server has a database in which it stores a copy of saidmessage key.
 30. The system of claim 29, wherein: said secondcomputerized system provides a recipient list to said key server; andsaid key server also stores a copy of said recipient list in saiddatabase.
 31. The system of claim 28, further comprising anauthentication server, and wherein said second computerized systemauthenticates the sender to said key server based on an assertion issuedby said authentication server, as a condition for said key serverproviding said message key to said second computerized system.
 32. Thesystem of claim 28, further comprising an authentication server, andwherein said key server asks said authentication server for either saidverifier or said secret, thereby permitting said key server to createsaid verifier.